1. Field of the Invention
The present invention relates to an image displaying and controlling apparatus and method and, more particularly, to an image displaying and controlling apparatus and method in which a roundness is assured when an image constructed of square-shaped pixels and having one aspect ratio is displayed as a second image constructed of rectangular-shaped pixels and having the other aspect ratio.
2. Description of the Related Art
Home television receivers or display monitors (NTSC television CRT display monitors are hereinafter simply referred to as TV monitors) have a large dot pitch and a slow screen refresh rate (30 frames per second). When an attempt is made to present on the TV monitor a graphics image produced in a personal computer, the presented image becomes coarser than the corresponding one on the display monitor of the personal computer. Thus, a proper image cannot be reproduced on the TV monitor.
If a TV monitor, possibly installed in a living room, is able to present a graphics image produced in a personal computer, both a television image and a computer graphics image can be presented using the same monitor. To this end, a VGA/NTSC scan converter is used. The scan converter first writes, onto a frame memory, image data fed by the personal computer, reads the image data at the vertical synchronizing frequency of the television receiver, and converts the image data into a composite video signal to be displayed on the TV monitor.
If the scan converter having the frame memory is built in a personal computer, the overall cost of the personal computer will be pushed up.
Alternatively, a personal computer may be equipped with a DVD (Digital Versatile Disc) player. The DVD player reproduces a bit stream in an MPEG (Moving Picture Expert Group) 2 format and the reproduced data is decoded through an MPEG2 decoder to be presented on a TV monitor.
In the above methods, the following problems will arise if a graphics image produced in the personal computer is overlaid on the image decoded by the MPEG2 decoder (referred to as MPEG2 image).
Digital coding rules of component signals in the current television system are formulated in Recommendation ITU-R (International Telecommunication Union Radio Communication Sector) BT. 601. This Recommendation specifies the sampling frequency, quantization level, and the like in the conversion of the analog signals of luminance and color difference into digital signals.
According to ITU-R BT. 601, the sampling frequency is 13.5 MHz, and the number of effective pixels for luminance signal per (scanning) line is 720. On the other hand, the NTSC Standard specifies that one frame is constructed of 525 lines. Out of these, the number of effective lines actually presented on screen is about 480.
According to MP@ML (MainProfile@Main Level), MPEG2, which is the International Standard ISO/ITC (International Organization for Standardization/International Electrotechnical Commission) 13818, 720 pixels/line, 576 lines/frame and 30 frames/s are specified.
The MPEG2 image is constructed of pixel data of 720.times.480 dots. The MPEG2 decoder alternately outputs an odd field image data constructed of 720.times.240 dots and an even field image data constructed of 720.times.240 dots in an interlace scanning system, at a rate of 60 fields a second. An MPEG2 image of 720.times.480 dots is thus displayed at a rate of 30 frames a second. Each dot constituting the image on screen is called a picture element or a pixel.
If an MPEG2 image of 720.times.480 pixels (having an aspect ratio of 3:2) is presented on the TV monitor having an aspect ratio of 4:3, the image is presented on screen with each pixel appearing as a rectangular pixel having its longer side vertically oriented.
Not all 720.times.480 pixels in the MPEG2 image are shown on the TV monitor, and an approximately 10% overscan area is provided in each of the vertical and horizontal directions. Actually visible on screen are about 648.times.432 pixels. FIG. 12A shows the relationship between a visible area and an image-present but invisible area (the overscan area).
The computer graphics image according to the well-known VGA (Video Graphics Array), which was formulated by IBM as the graphics standard for IBM PC AT and their compatible machines, includes 640.times.480 pixels in one frame. As shown in FIG. 12B, the VGA image is presented on screen with its all pixel visible on the display monitor.
If the VGA image is presented on the TV monitor having an aspect ratio of 4:3, each pixel is presented as a square pixel because the image of 640.times.480 pixels constituting the VGA image is equivalently a 4:3 aspect image.
As shown in FIG. 11, when the MPEG2 image of 720.times.480 pixels is presented on the TV monitor is mixed with the VGA image of 640.times.480 pixels in an overlay fashion, the VGA image is shown elongated vertically because of the aspect ratio difference. In other words, the roundness of image is not 1.
Referring to FIG. 13, the VGA image of 640.times.480 pixels is line-number converted from 480 lines to 432 lines so that the VGA image has the same aspect ratio of 4:3 as that of the MPEG2 image, and is then mixed with the MPEG2 image. In this way, the VGA image is presented in the regular roundness.
When the VGA image is presented after being converted from 480 lines to 432 lines, the number of lines has to be halved to 216 to be compatible with the interlace scanning system.
Referring to FIG. 14, removing the top 12 lines and bottom 12 lines from one field MPEG2 image constructed of 240 lines results in 216 lines, and if one field VGA image in the interlace scanning system is presented over the 216 lines, the VGA image will be overlaid on the MPEG2 image in the regular roundness.
These 216 lines are generated by processing the image data of the lines in the vicinity of the 480 lines in a VGA image in a non-interlace scanning system. Suppose that the scanning of the non-interlaced VGA image is performed twice the rate of scanning of the NTSC system, the non-interlaced VGA image is scanned at a rate of 480 lines per field. The interlaced VGA image of 216 lines per field may be obtained by processing a non-interlaced VGA image of 480 lines.
As shown in FIG. 14, the lines within an area of r of the 216 lines in an interlaced VGA image may be produced from the lines within an area R of the 480 lines in a non-interlaced VGA image. As can be seen from FIG. 14, at the timing of producing the lines within the area r, the lines within the area R are not yet supplied. For this reason, the non-interlaced VGA image is once stored in a frame memory, and out of the image stored, the image data corresponding to the lines within the area is read to produce the lines of the interlaced VGA image.
The use of a frame memory for line number conversion pushes up the cost of the apparatus.